Wireless communication standards are continuously changing in order to adapt to the high volume data transfer required today between consumers. As a result, operators of wireless communication systems struggle with extra costs that result from upgrading the wireless system or the complete replacement of the already deployed sites. Also, the continuously changing of wireless communication standards disturbs base station vendors in their product strategy and portfolio. Multi-Standard or Multi-Band Radio Base Station is a solution that may reduce the deployment cost for the operators as well as the production cost for the network infrastructure component vendors. Particularly, a transceiver power amplifier is one of the most critical components in a wireless base station. Today's Power Amplifier is not only required to be highly efficient in order to keep the base station Operating Expense (OPEX) low but must be also broadband enough to operate in a Multi-band system.
Because of its simplicity, the Doherty Power Amplifier (PA) is the most efficient architecture that is used today to efficiently transmit high peak to average ratio modulated signals. However, the Doherty PA is inherently narrowband and its performance is significantly reduced when used in a multi-band transceiver. In fact, the radio frequency bandwidth (BW) of the Doherty PA is not only limited by the individual stage BW (Main stage and Peak stage) and the output quarter-line combiner, but also the BW of the Doherty PA is limited by the inherent phase linearity of the transmission lines that constitute the matching networks and/or the offset lines used at the output of the main and peak stages.
For example, the conventional Doherty amplifier uses two amplifier stages, the main amplifier and the peak amplifier, with their outputs interconnected by a quarter wavelength impedance inverter. The Doherty amplifier maintains its maximum efficiency at power back-off with the help of the peak amplifier and the quarter-wavelength transformer. At power back-off, the peak amplifier is intentionally kept OFF to allow the saturation of the main amplifier by doubling its load impedance through the impedance transformer. The BW of the Doherty amplifier is related to the BW of the individual stages and to the output quarter wavelength transformer BW. Also, at average operating power, the Doherty BW is narrower than expected even with using broadband main and peaking stages. This is due to the fact that the matching network transmission line phase and/or the offset lines phase are inherently linear over frequency, which inherently limits the BW of the Doherty Amplifier.
This significantly degrades the efficiency and the power at the frequency band edges. The same applies for the Peaking OFF impedance, which is real and high at only one frequency (good isolation only at mid-band). Hence, this drawback limits the Doherty high efficiency operating BW to a max of 5-7%. The effect of the output quarter wavelength inverter on the BW starts to be significant for BW higher than 10%. To be able to be used in a multi-standard transceiver, the Doherty power amplifier is required to be tunable over the whole frequency bands of interest.